1. Technical Field of the Invention
The present invention relates to a method and an apparatus for powder compression molding for manufacturing cathode pellets for batteries.
2. Description of Related Art
Molding of powder mixture pellets is generally accomplished with a rotary type compression molding apparatus, wherein a rotary disk carries at its circumferential edge a number of molding units arranged at equal intervals, and as the rotary disk rotates, the molding units successively perform molding actions by means of cam members. FIG. 8 shows one example of prior art arrangement for one such molding unit mounted on a rotary disk. A center pin 34 passing through a lower plunger 35 is fixed to the rotary disk 31 with a fixing pin 44. An upper plunger 36 has a cylindrical cavity to receive therein the top end of the center pin 34. The upper plunger 36 and the lower plunger 35 are respectively pressed in molding directions with an upper pressure roller 39 and a lower pressure roller 38 at predetermined locations on the rotary disk.
FIGS. 9A to 9E illustrate the process steps for molding ring-like pellets with the molding unit of FIG. 8. FIG. 9A illustrates an initial state where a die 33, the lower plunger 35, and the center pin 34 are flush with each other at the top after the previous pellet has been unloaded. When the lower plunger 35 is lowered from its initial position, an annular space for compression molding is formed between the die 33 and the center pin 34 as shown in FIG. 9B. The annular space is then filled with a powder mixture 42. As a feed shoe 43 runs along the top sides of the die 33 and the center pin 34 located flush with each other, an excess of the powder mixture 42 is removed to measure out a predetermined amount to be molded into one pellet 40. This is followed by a step where the upper plunger 36 is lowered and the lower plunger 35 is lifted up as shown in FIG. 9D, by which the powder mixture 42 in the annular space is compressed from upper and lower sides, thus forming the pellet 40. The pellet 40 is then unloaded upwardly from the die 33 by the upward movement of the lower plunger 35 as shown in FIG. 9E and taken out as a compression molded product.
Such conventional procedure of compression molding has, however, a drawback that the powder mixture 42 when being supplied into the annular space between the die 33 and the center pin 34 is likely to produce a bridge across the very small gap between the center pin and the die, particularly when a thin pellet 40 having a small diameter is formed. Because of the bridges frequently formed, it is difficult to constantly supply a given amount of the powder mixture 42, resulting in variations in the weight of pellets 40.
In view of the stringent requirements for uniform and high electrical performance of batteries in recent years, it is extremely important to reduce variations in the battery capacity which is directly determined by the amount of pellets forming the active material of battery. Therefore, it is essential to ensure that each pellet is molded from a predetermined, constant amount of powder mixture, so that pellets are formed with as little variation as possible in their weight and volume.
Another problem associated with the conventional molding apparatus is that the components constituting the cam mechanism for driving the plungers are subject to great stress, and particularly small components, such as the fixing pin 44 in FIG. 8 for fixing the center pin 34, which cannot withstand the stress, often break and must be replaced. For the same reasons the sliding surfaces between the upper and lower plungers and the pressure rollers, and the cams which come to frequent engagement therewith suffer severe abrasion. These are all because the pellet 40 is tightly stuck to the center pin 34 and the die 33 by the pressure given during the compression molding, and a great amount of force is required to push the formed pellet out of the die 33.
Also, since the powder mixture is compressed from the upper and lower sides, the adhesion between the inner side of the resultant pellet and the center pin and that between the outer side of the pellet and the die 33 is very strong. In order to unload the pellet with the lower plunger 35 without damaging the pellet 40, it is the normal practice to provide a tapered surface at a relatively wide angle to the center pin 34 and to provide, correspondingly, an inverted tapered surface to the die 33. Therefore, the resultant pellets 40 are tapered on both inner and outer sides. The amount of the powder mixture contained in one battery is therefore reduced by these tapered portions.
Under the circumstances, several ring-like, tapered pellets are contained in one cell as shown in FIG. 10. As shown, one cell 61 normally contains four ring-like pellets 40 made from a powder mixture consisting of positive electrode active materials because of the small height of pellets. Negative electrode active material is contained within the cylindrical bore hole formed by these ring-like pellets arranged vertically upon one another in a cylindrical cell case, with a separator 64 interposed therebetween.
With such structure, there is still room for more active material to be filled because of the taper as mentioned above. Also, the tapered surfaces of pellets create slight gaps between themselves and the separator 64 and the cell case 62 as can be seen from FIG. 10, because of which smooth flow of electric current is obstructed. Moreover, since the cells need to be filled with four pellets each, the production line involves a considerable number of operations, resulting in low efficiency and high cost. Therefore, it is extremely desirable that pellets for batteries be formed with a greater height and with as little taper as possible.
Usually, ring-like pellets produced by the compression molding machine as described above are transferred by belt conveyors or parts feeders to a next step in which they are loaded into cell cases. However, since the molded pellets tend to be broken at their edges during transportation, those steps are hardly carried out at a high speed. Specifically, the pellets for alkaline manganese dry cells are composed of a mixture of manganese dioxide and graphite with a binder, and these are extremely fragile and liable to chipping even with a slight shock. Also, the entire system including the compression molding machine, conveyors, and automatic loaders is bulky and not adaptable for mass production at high speed.
These problems have previously been addressed by packing a powder mixture in a cell case and compression-molding it within the cell case, as disclosed in U.S. Pat. Nos. 3,577,842 and No. 3,729,281. With such methods, while a strong bond is obtained between the case and the shaped mold of the powder, the density of the mold thus shaped is relatively low, because it is molded within the confined space of cell case from which air cannot escape, and because the powder is compressed from only one direction, i.e., from the open end side of the cell cases. Also, because the center pin inherently has a flat top end, there is a problem that powder sticks thereto. Moreover, the center pin must have a tapered surface for facilitating removal from the shaped mold, which presents the disadvantages described above.
In view of the foregoing, it is an object of the present invention to provide an improved powder compression molding method and an apparatus for producing high and hollow cylindrical pellets, of which inner side is not tapered at all and of which outer side is substantially not tapered.
Another object of the present invention is to improve measurement precision of powder mixture to be molded into pellets, so that resultant pellets have as little variation as possible in weight.
It is yet another object of the present invention to provide a compact rotary type powder compression molding assembly system that is adaptable for mass production and high speed production.
To accomplish the above-mentioned objects, the present invention provides a powder compression molding method comprising the steps of:
lowering a lower plunger and a center pin both mounted coaxially in a cylindrical die to a first position;
supplying a powder into the cylindrical die;
lifting the lower plunger and the center pin to a second position;
removing an extra of the powder to cause a given amount of the powder corresponding to one pellet to remain in the die;
lowering an upper plunger to close the die at a top end thereof with a lower end of the upper plunger;
lifting up the center pin to extend through the powder, thereby defining an annular molding space between itself and the cylindrical die;
compressing the powder in the annular space with the upper plunger and the lower plunger;
lifting up the center pin simultaneously with the lower plunger to push a formed pellet upwardly to the outside; and
withdrawing the center pin from the pellet after the center pin and the pellet have been transferred from the annular molding space.
According to the method of the present invention, since the center pin mounted coaxially within the cylindrical die and arranged movable is lowered before the powder is supplied into the die, there is no risk that bridges of powder are formed within the die. The center pin may be vertically reciprocated several times during the step of supplying the powder mixture into the die, so that any bridges that may be formed within the confined space in the die are destroyed. Moreover, after the powder is supplied and the heap of the powder is leveled with a shoe along the top surface of the die so that the die is filled precisely with a predetermined amount of powder, the lower plunger is lowered to cause the powder to sink to a position lower than the top surface of the die. The upper plunger is then lowered to close the upper open end of the die, so that the precisely measured amount of powder remains in the die. The center pin is then lifted up to its molding position, and on its way upwards, the center pin is vertically reciprocated a plurality of times. In this process, any powder that may have remained on the tip of the center pin is completely removed. Thus the annular molding space can be filled with a constant amount of the powder mixture, whereby pellets of uniform weight and density can be formed stably.
After the compression molding of the powder mixture, the center pin is simultaneously lifted up with the lower plunger so as to push up the formed pellet out of the die, after which the center pin is withdrawn from the pellet. Since the formed pellet is subject to a large compressive force after the molding in the die, there is strong adhesion between the inner surface of the pellet and the center pin and, if an attempt were to be made to eject the pellet only by the action of the lower plunger, it would be necessary to apply a considerable amount of force to the upward movement of the lower plunger, resulting in the problems mentioned above. Also, a wide-angled taper would have to be provided to both of the center pin and the die to facilitate the removal of the pellet. The present invention provides a method which solves all of these problems, in which the center pin is controlled to be lifted together with the lower plunger with the formed pellet still held thereon. Thereby, the formed pellet can be taken out of the die with a much smaller amount of force. The pellet, when moved to the outside, is released from the compressing stress of the die, whereupon the contact between the inner surface of the pellet and the center pin is lessened, as a result of which the center pin can be readily withdrawn from the pellet afterwards. Accordingly, the center pin need not have a tapered surface as in the prior art, and the amount of powder per one pellet can be increased by the tapered portion which was previously necessary.
The tubular pellets thus formed are very thin and high, and must be handled with great care. According to the method of the present invention, pellets are inserted into a cylindrical container which is held in position over and in alignment with the cylindrical die immediately after the molding. Each cylindrical container is held by respective convey jigs throughout the process, ensuring that the containers do not fall or tilt. Therefore there is no risk that the formed pellets are physically damaged, and molding and assembling of pellets for batteries can be accomplished simply at low cost.
In order to implement the above described method, the present invention provides a powder compression molding apparatus comprising:
a molding unit including a cylindrical die, a center pin disposed at the axial center of the die, and a lower plunger and an upper plunger for compressing a powder mixture supplied in an annular molding space defined between the die and the center pin into a ring configuration, the lower plunger and the center pin being arranged to be movable in the axial direction in relation to each other as well as to the die in a powder fill mode and to further eject cooperatively the molded ring configuration into a cylindrical container;
an upper lifting shaft, to the lower end of which the upper plunger is secured;
an upper plunger actuating means including a first upper plunger cam follower detachably coupled to the upper end of the upper lifting shaft, and a pressure roller engaging with the first upper plunger cam follower, for driving the upper plunger to perform a compression molding action in the die;
a second upper plunger cam follower connected to the upper lifting shaft for lowering the upper plunger to close the upper open end of the die with the lower end of the upper plunger prior to the compression molding action;
a lower lifting shaft, to the upper end of which the lower plunger is secured and within which the center pin is coaxially disposed for relative sliding movements, the lower lifting shaft including a hollow cavity and supports therein a biasing spring to bias the center pin upward;
a lower plunger actuating means including a lower plunger cam follower detachably coupled to the lower end of the lower lifting shaft and a pressure roller engaging with the lower plunger cam follower for driving the lower plunger to perform the compression molding action in the die together with the upper plunger;
a cam for carrying out an ejecting operation to move the lower plunger upwardly to push the molded ring configuration out of the die;
a center pin actuating means including a center pin cam follower connected to the center pin through the lower lifting shaft, and a center pin actuating cam engaging with the center pin cam follower for moving the center pin upwardly to the molding position after being moved downwardly to locate lower than the top surface of the die, and for carrying out the ejecting operation to move the center pin upwardly simultaneously with the lower plunger.
The center pin actuating cam constituting the center pin actuating means has a cam surface only at a lower side thereof, and the biasing spring detachably presses the center pin cam follower connected to the center pin against the cam surface of the center pin actuating cam. Thus, the center pin cam follower is urged upward towards the cam surface of the center pin actuating cam, so as to enable the center pin to perform each action sensitively in accordance with the configuration of the cam surface. Moreover, any downward stress exerted to the center pin is absorbed by the contracting action of the spring and does not directly act on the center pin. This permits the lower part of the center pin to be of a reduced diameter and to remain intact as well as preventing any damage or bending to the cam follower shaft. Furthermore, the center pin actuating means is configured such as to cause the center pin to vertically reciprocate more than once before forcing an upper end of the center pin above the powder mixture in a pre-mold preparatory operation.
As mentioned above, because at least the inner side of the pellet need not be tapered, the center pin has a straight side face, and a pointed tip, in order to facilitate removal of any remnants of powder thereon.